The present invention relates to a mechanism for driving capstans in a tape recorder.
One known mechanism for driving a pair of capstans in a tape recorder includes a pair of flywheels of equal diameters which are attached to the capstans, respectively, and rotated by an endless belt driven by the pulley of a capstan motor. A magnetic tape is fed along when it is held against the capstans by pinch rollers or idlers, respectively, when the capstan motor is energized. During operation of the capstan mechanism, the endless belt, when leaving the one of the flywheels which is positioned downstream of the other in the direction of feed of the magnetic tape, tends to be tensioned since it is directly pulled by the motor pulley. On the other hand, the endless belt as it is to engage the other flywheel is liable to sag since the endless belt has just left the motor pulley. Therefore, the capstan on the downstream flywheel rotates slightly faster than the other capstan, thus tensioning a portion of the magnetic tape between the capstans. When the rotation of the motor is reversed to feed the magnetic tape backward, the magnetic tape portion between the capstans is also tensioned as the capstans rotate at slightly different speeds due to varied tensioning of the endless belt for the reasons described above. The tension thus imposed on the magnetic tape however can easily vary when the endless belt becomes permanently stretched or has its contact surface smeared or otherwise damaged.
There has been proposed a mechanism including a pair of capstans having different diameters or a pair of flywheels having different diameters for tensioning a magnetic tape by rotating the capstans at different speeds, that is, slower at the upstream capstan and higher at the downstream capstan. While the proposed arrangement is effective in running the magnetic tape at relatively constant tension, the speeds of rotation of the capstans must remain unchanged, and hence the magnetic tape cannot be fed in a reverse direction as by reversing the rotation of the capstan motor.